The reaction products occurring, for example, in the production of polysilicon generally contain hydrogen, chlorosilanes and hydrogen chloride. These can, particularly in moist air, have a highly corrosive action and can additionally be spontaneously flammable due to a proportion of polymeric silicon-hydrogen chloride compounds. The offgases are therefore usually subjected to a work-up in which hazardous components react and are converted into nonhazardous or less hazardous components. Such a work-up can, for example, encompass hydrolysis steps in which a chemical compound is dissociated by reaction with water. Chlorosilanes can be removed by treatment with water, for example to form their hydrolysis products (e.g. RSi(OH)3), and be separated off.
Particularly in the preparation of trichlorosilane (TCS), which serves as starting material for the production of polysilicon, from hydrogen chloride and metallurgical silicon, a fraction comprising chlorosilicon compounds whose boiling range is basically from about 40 to 160° C. is usually formed. This fraction, which is generally referred to as high boiler or high boiler mixture, can comprise chlorodisilanes, chlorodisiloxanes, metal chlorides, dopants and also trichlorosilanes and tetrachlorosilanes. Since some of the compounds mentioned can have similar boiling points, separation of such high boilers by distillation is generally associated with a large outlay. For this reason, the high boilers are separated off from the other chlorosilanes and subsequently reacted, usually by means of hydrolysis.
DE 28 20 617 A1 discloses, for example, a method in which the hydrolysis of a high boiler mixture is carried out in a tube (hydrocyclone) which tapers conically in a downward direction. The high boiler mixture comprises chlorosilanes. Water is fed in tangentially from above, at the top of the hydrocyclone, as a result of which a rotating cone of water is formed. The liquid high boiler mixture is sprayed together with an inert carrier gas through a two-fluid or two-channel nozzle into the cone of water, likewise at the top of the hydrocyclone. Rapid and uniform reaction can be ensured by the large surface area of water and the fine dispersion of the high boiler mixture in the form of the spray jet.
However, interruptions to the hydrolysis process can occur in the case of such methods. These interruptions can, in particular, occur as a result of blockages and deposits in the form of solid or gel-like silica in the region of the two-fluid nozzle and the conical tube. Particularly in the region close to the nozzle, blockages can occur as a result of contact of the finely dispersed (atomized) high boilers with water vapor. Cleaning work frequently has to be carried out, and this results in lower plant availability and thus higher production costs. It can be quite normal for a hydrocyclone as per the above example to have to be subjected to cleaning on average once per day.
WO 2009/037923 A1 describes a device for vapor-phase hydrolysis in which the compound to be hydrolyzed is mixed with water and an inert carrier gas in a combustion space by means of a three-fluid or three-channel nozzle. The mixing and thus the hydrolysis of the compound occurs mainly at the nozzle outlet.
However, occurrence of recirculation zones (turbulences) in the region downstream of the nozzle can occur in the device described. Deposits preferentially arise in these zones of low flow velocity since carrying-away of the hydrolysis products formed is no longer ensured. Furthermore, the hydrolysis is carried out in a temperature range from 850 to 1100° C. Operation of the device in this temperature range requires a considerable engineering outlay (especially cooling and heating).